Porous electrode substrate and process for production thereof, porous electrode substrate precursor sheet, membrane-electrode assembly, and polymer electrolyte fuel cell

ABSTRACT

Provided are: a porous electrode substrate which has excellent handling properties and surface smoothness and satisfactory gas permeability and electrical conductivity, and enables the reduction of damage to a polymer electrolyte membrane when integrated into a fuel cell; and a process for producing the porous electrode substrate. Specifically provided are: a porous electrode substrate comprising a three-dimensional structure (Y-1) produced by bonding short carbon fibers through carbon and a three-dimensional structure (Y-2) produced by bonding short carbon fibers through carbon, wherein the three-dimensional structures (Y-1) and (Y-2) are layer stacked on and integrated with each other, the short carbon fibers form a three-dimensional entangled structure in the structure (Y-1), and the short carbon fibers do not form a three-dimensional entangled structure in the structure (Y-2); a process for producing the electrode base material; a precursor sheet for producing the electrode base material; a membrane-electrode assembly which involves the electrode base material; and a polymer electrolyte fuel cell.

TECHNICAL FIELD

The present invention relates to a process of producing a porouselectrode substrate used in polymer electrolyte fuel cells.

BACKGROUND ART

Polymer electrolyte fuel cells are characterized in using a protonconductive polymer electrolyte membrane, and are devices that obtainelectromotive force by allowing oxidizing gas such as oxygen toelectrochemically react with fuel gas such as hydrogen. Such polymerelectrolyte fuel cells have a structure in which two gas diffusionelectrodes, including a catalyst layer with carbon powder on which anoble metal-based catalyst is supported as a main component and a gasdiffusion electrode base material, respectively have a catalyst layerside thereof set to the inside, and are bonded to both sides of apolymer electrolyte membrane.

The gas diffusion electrode substrate is generally configured from acarbonaceous material, and the gas diffusion electrode substratesindicated below are known, for example.

Patent Document 1 discloses a porous carbon electrode substrate for fuelcells characterized by having a thickness of 0.05 to 0.5 mm, a bulkdensity of 0.3 to 0.8 g/cm³, and a bending strength of at least 10 MPaand deflection upon bending of at least 1.5 mm in a three-point bendingtest under the conditions of a strain rate of 10 mm/min, a distancebetween support points of 2 cm and a test piece width of 1 cm.

Patent Document 2 discloses a gas diffusion layer for fuel cellsincluding a mat having a plurality of carbon fibers, and a plurality ofacrylic pulp fibers incorporated into this carbon fiber mat, and thatthe acrylic pulp fibers are cured and carbonized after incorporationinto the carbon fiber mat.

Patent Document 3 discloses a carbon fiber sheet having a thickness of0.15 to 1.0 mm, a bulk density of 0.15 to 0.45 g/cm³, a carbon fibercontent of at least 95% by mass, a compressive deformation rate of 10 to35%, an electrical resistance value of no more than 6 mΩ, and a degreeof drape of 5 to 70 g.

Patent Document 4 discloses a carbon fiber non-woven fabric for apolyelectrolyte fuel cell electrode material having a thickness of 0.15to 0.60 mm, a basis weight of 50 to 150 g/m², a specific resistancevalue in the thickness direction of no more than 0.20 Ωcm, and a surfacepile number of no more than 15/mm².

-   Patent Document 1: Pamphlet of PCT International Publication No.    WO2001/056103-   Patent Document 2: Japanese Unexamined Patent Application,    Publication No. 2007-273466-   Patent Document 3: Pamphlet of PCT International Publication No.    WO2002/042534-   Patent Document 4: Japanese Unexamined Patent Application,    Publication No. 2003-45443

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, although the porous carbon electrode material disclosed inPatent Document 1 has high mechanical strength and surface smoothness,and has adequate gas permeability and electrical conductivity, theproduction cost thereof is high. Although the gas diffusion layer forfuel cells disclosed in Patent Document 2 can be produced at low cost,there is little entangling of the acrylic pulp with carbon fibers uponmaking into a sheet, and thus the handling thereof has been difficult.In addition, since acrylic pulp has low molecular orientation of polymercompared with fibrous materials, the carbonization rate duringcarbonization is low, and thus it has been necessary to add an abundanceof acrylic pulp in order to raise the handling property. The carbonfiber sheet and carbon fiber non-woven fabric disclosed in PatentDocuments 3 and 4 can be produced at low cost; however, the shrinkagethereof during calcining is large, and thus the undulation (heavingstate or warped state of sheet cross section) of the obtained sheet,etc. has been large. Furthermore, since the handling property isimproved by fiber entangling, the sheet surface has become fluffy fromthe carbon fibers, whereby the polymer electrolyte membrane has beendamaged upon incorporating the sheet in the fuel cell.

The present invention has an object of providing a porous electrodesubstrate excelling in handling property, having improved sheetundulation, as well as retaining sufficient gas permeability andelectrical conductivity, and further, does not inflict damage on thepolymer electrolyte membrane upon incorporating in the fuel cell, aswell as a process of producing the same.

In addition, the present invention has an object of providing a porouselectrode substrate precursor sheet that can be suitably used in orderto obtain the above-mentioned porous electrode substrate, as well as amembrane-electrode assembly and polymer electrolyte fuel cell using theabove-mentioned porous electrode substrate.

Means for Solving the Problems

The following first to twelfth aspects of the invention are provided bythe present invention.

According to a first aspect of the invention, a porous electrodesubstrate includes, layer stacked and integrated therein:

a three-dimensional structure Y-1 produced by bonding short carbonfibers (A1) by carbon (D); anda three-dimensional structure Y-2 produced by bonding short carbonfibers (A2) by carbon (D),in which the short carbon fibers (A1) form a three-dimensional entangledstructure in the three-dimensional structure Y-1, and the short carbonfibers (A2) do not form a three-dimensional entangled structure in thethree-dimensional structure Y-2.

According to a second aspect of the invention, in the porous electrodesubstrate as described in the first aspect, the three-dimensionalstructure Y-1 is a three-dimensional structure in which the short carbonfibers (A1) are bonded by three-dimensional mesh-like carbon fibers (B),and

the three-dimensional structure Y-2 is a three-dimensional structure inwhich the short carbon fibers (A2) are bonded by two-dimensionalmesh-like carbon fibers (C).

According to a third aspect of the invention, a porous electrodesubstrate precursor sheet includes, layer stacked and integratedtherein: a precursor sheet X-2′ having dispersed therein short carbonfibers (A1), and short carbon fiber precursors (b1) and/or fibrillarcarbon fiber precursors (b1′); and

a precursor sheet X-3′ having dispersed therein short carbon fibers(A2), and short carbon fiber precursors (b2) and/or fibrillar carbonfiber precursors (b2′),in which the short carbon fibers (A1) form a three-dimensional entangledstructure in the precursor sheet X-2′, andthe short carbon fibers (A2) do not form a three-dimensional entangledstructure in the precursor sheet X-3′.

According to a fourth aspect of the invention, in the porous electrodesubstrate as described in the third aspect, amass ratio of a mass of theshort carbon fibers (A1) to a total mass of the short carbon fiberprecursors (b1) and the fibrillar carbon fiber precursors (b1′) is 20:80to 80:20, and

a mass ratio of a mass of the short carbon fibers (A2) to a total massof the short carbon fiber precursors (b2) and the fibrillar carbon fiberprecursors (b2′) is 20:80 to 80:20.

According to a fifth aspect of the invention, a process of producing aporous electrode substrate includes: a step (1) of dispersing shortcarbon fibers (A1), and producing a precursor sheet X-1 of the shortcarbon fibers (A1) not having a three-dimensional entangled structure;

a step (2) of obtaining a precursor sheet X-2 having a three-dimensionalentangled structure of the short carbon fibers (A1), by entanglementtreating the precursor sheet X-1;a step (3) of obtaining a porous electrode substrate precursor sheet X-4by dispersing short carbon fibers (A2), and layer stacking andintegrating a precursor sheet X-3 not having a three-dimensionalentangled structure of the short carbon fibers (A-2), on the precursorsheet X-2; anda step (4) of carbonization treating the porous electrode substrateprecursor sheet X-4 at a temperature of at least 1000° C.

According to a sixth aspect of the invention, in the process ofproducing a porous electrode substrate as described in the fifth aspect,the precursor sheet X-2 having a three-dimensional entangled structureof the short carbon fibers (A1) and the precursor sheet X-3 not having athree-dimensional entangled structure of the short carbon fibers (A2)are layer stacked and integrated in step (3), by feeding a slurry inwhich the short carbon fibers (A2) are dispersed in a liquid medium ontothe precursor sheet X-2 and making into a sheet.

According to a seventh aspect of the invention, in the process ofproducing a porous electrode substrate as described in the fifth orsixth aspect, the short carbon fiber precursors (b1) and/or fibrillarcarbon fiber precursors (b1′) are dispersed together with the shortcarbon fibers (A1) in step (1), and the short carbon fiber precursors(b2) and/or fibrillar carbon fiber precursors (b2′) are dispersedtogether with the short carbon fibers (A2) in step (3).

According to an eighth aspect of the invention, the process of producinga porous electrode substrate as described in any one of the fifth toseventh aspects further includes a step (5) of hot press molding theporous electrode substrate precursor sheet X-4 at a temperature of lessthan 200° C., after step (3) but before step (4).

According to a ninth aspect of the invention, the process of producing aporous electrode substrate as described in the eighth aspect furtherincludes a step (6) of oxidation treating, at a temperature of at least200° C. and less than 300° C., the porous electrode substrate precursorsheet X-4 subjected to hot press molding, after step (5) but before step(4).

According to a tenth aspect of the invention, a porous electrodesubstrate is obtained by the process as described in any one of thefifth to ninth aspects.

According to an eleventh aspect of the invention, a membrane-electrodeassembly includes the porous electrode substrate as described in thefirst, second or tenth aspect.

According to a twelfth aspect of the invention, a polymer electrolytefuel cell includes the membrane-electrode assembly as described in theeleventh aspect.

Effects of the Invention

The present invention provides a porous electrode substrate excelling inhandling property, having improved sheet undulation, as well asretaining sufficient gas permeability and electrical conductivity, andfurther, does not inflict damage on the polymer electrolyte membraneupon inserting in the fuel cell, as well as a process of producing thesame.

In addition, the present invention provides a porous electrode substrateprecursor sheet that can be suitably used in order to obtain theabove-mentioned porous electrode substrate, as well as amembrane-electrode assembly and polymer electrolyte fuel cell using theabove-mentioned porous electrode substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron micrograph of a cross section of athree-dimensional structure of a porous electrode substrate obtained inExample 1 having a three-dimensional entangled structure;

FIG. 2 is a scanning electron micrograph of a cross section of athree-dimensional structure of a porous electrode substrate obtained inExample 1 not having a three-dimensional entangled structure;

FIG. 3( a) is a view illustrating an observation view (A surface) of theporous electrode substrate obtained in Example 1, and (b) is a scanningelectron micrograph of the A surface;

FIG. 4( a) is a view illustrating an observation view (B surface) of theporous electrode substrate obtained in Example 1, and (b) is a scanningelectron micrograph of the B surface;

FIG. 5 is an image arrived at by drawing a dotted line on carbon fibersto be measured in the scanning electron micrograph shown in FIG. 1, inorder to measure the angle between a short carbon fiber and a horizontalsurface;

FIG. 6 is an image arrived at by drawing a dotted line drawn on carbonfibers to be measured in the scanning electron micrograph shown in FIG.2, in order to measure the angle between a short carbon fiber and ahorizontal surface;

FIG. 7 is a schematic diagram of a three-dimensional structure in whichshort carbon fibers (A1) are bonded by three-dimensional mesh-likecarbon fibers (B); and

FIG. 8 is a perspective view of a three-dimensional structure not havinga three-dimensional entangled structure, in which short carbon fibers(A2) are connected by two-dimensional mesh-like carbon fibers (C).

EXPLANATION OF REFERENCE NUMERALS

-   -   1: line parallel to sheet surface    -   2: carbon fiber constituting three-dimensional mesh-like carbon        fiber (B)    -   3: carbon fiber constituting two-dimensional mesh-like carbon        fiber (C)    -   A1: short carbon fibers    -   A2: short carbon fibers

PREFERRED MODE FOR CARRYING OUT THE INVENTION Porous Electrode Substrate

A porous electrode substrate of the present invention is composed of astructure layer stacking and integrating a three-dimensional structureY-1 made by bonding short carbon fibers (A1) by carbon (D) and athree-dimensional structure Y-2 made by bonding short carbon fibers (A2)by carbon (D). The short carbon fibers (A1) form a three-dimensionalentangled structure in the three-dimensional structure Y-1. The shortcarbon fibers (A2) do not form a three-dimensional entangled structurein the three-dimensional structure Y-2. It should be noted that theshort carbon fibers (A1) and short carbon fibers (A2) (hereinafter maybe collectively called “short carbon fibers (A)”) may be the same or maybe different.

The three-dimensional structure Y-1 is a three-dimensional structuremade by bonding the short carbon fibers (A1) by carbon (D), and is astructure in which the short carbon fibers (A1) constituting thestructure Y-1 are three-dimensionally entangled in the structure Y-1.

In addition, the three-dimensional structure Y-2 is a three-dimensionalstructure made by bonding the short carbon fibers (A2) by carbon (D),and is a structure in which the short carbon fibers (A2) constitutingthe structure Y-2 are not three-dimensionally entangled in the structureY-2.

The three-dimensional structure Y-1 may be a three-dimensional structurein which the short carbon fibers (A1) are bonded by thethree-dimensional mesh-like carbon fibers (B), and the three-dimensionalstructure Y-2 may be a three-dimensional structure in which the shortcarbon fibers (A2) are bonded by two-dimensional mesh-like carbon fibers(C).

The porous electrode substrate in which the three-dimensional structureY-1 and the three-dimensional structure Y-2 are layer stacked andintegrated can be in forms such as a sheet form and a spiral form. In acase of making in sheet form, the basis weight of the porous electrodesubstrate is preferably on the order of 15 to 100 g/m², the porosity ispreferably on the order of 50 to 90%, the thickness is preferably on theorder of 50 to 300 μm, and the undulation is preferably no more than 5mm. The gas permeability of the porous electrode substrate is preferably50 to 3000 ml/hr/cm²/Pa. In addition, the electrical resistance(through-plane electrical resistance) in the thickness direction of theporous electrode substrate is preferably no higher that 50 mΩ·cm². Itshould be noted that the method of measuring the gas permeability andthrough-plane electrical resistance of the porous electrode substratewill be described later.

The total content of the three-dimensional mesh-like carbon fiber (B) totwo-dimensional mesh-like carbon fiber (C) in the porous electrodesubstrate is preferably 5 to 90% by mass, and more preferably 10 to 60%by mass, from the viewpoint of the mechanical strength of the porouselectrode substrate. In other words, the content of short carbon fiber(A) in the porous electrode substrate is preferably 10 to 95% by mass,and more preferably 40 to 90% by mass.

Presence or Absence of Three-Dimensional Entangled Structure

In the present invention, whether or not the short carbon fibers (A)form a three-dimensional entangled structure can be determined byperforming cross-sectional observation of the sheet-like measurementtarget (three-dimensional structure Y-1, three-dimensional structureY-2, porous electrode substrate, precursor sheet X-2′, precursor sheetX-3′, porous electrode substrate precursor sheet, precursor sheet X-1,precursor sheet X-2, precursor sheet X-3), and measuring the anglesbetween the respective short carbon fibers and the sheet surface in thecross section. It should be noted that the cross section in whichcross-sectional observation is made is a cross section in a verticaldirection relative to the sheet surface of a sheet-like measurementtarget.

In a case of the average of the angles of the short carbon fibers withthe horizontal plane (sheet surface) measured being at least 3°, or themaximum value of the angle between the short carbon fibers and thehorizontal plane being at least 10°, it is determined that the shortcarbon fibers are forming a three-dimensional entangled structure (themeasurement target has a three-dimensional entangled structure), and ina case of not being as such, it is determined that the short carbonfibers are not forming a three-dimensional entangled structure (themeasurement target does not have a three-dimensional entangledstructure). More specifically, using an SEM image of a cross section inthe vertical direction relative to the sheet surface and drawing linesas indicated by the dotted lines on the short carbon fibers to bemeasured as in FIGS. 5 and 6, it is sufficient to measure the anglebetween this line and the sheet surface (The lines 1 in FIGS. 5 and 6are lines parallel to the sheet surface.). It should be noted that thenumber of measurement points upon deciding the average value and maximumvalue of angles can be set to 50 points, for example.

Three-Dimensional Structure in which Short Carbon Fibers (A1) are Bondedby Three-Dimensional Mesh-Like Carbon Fibers (B)

A schematic diagram of a three-dimensional structure in which the shortcarbon fibers (A1) are bonded by the three-dimensional mesh-like carbonfibers (B) is shown in FIG. 7. In this three-dimensional structure, theshort carbon fibers (A1) are bonded by each of the carbon fibers 2constituting the three-dimensional mesh-like carbon fibers (B).

Three-Dimensional Structure in which Short Carbon Fibers (A2) are Bondedby Two-Dimensional Mesh-Like Carbon Fibers (C)

A schematic diagram of a three-dimensional structure in which the shortcarbon fibers (A2) are bonded by the two-dimensional mesh-like carbonfibers (C) is shown in FIG. 8. In this three-dimensional structure, theshort carbon fibers (A2) are bonded by the respective carbon fibers 3constituting the two-dimensional mesh-like carbon fibers (C).

Determination of Mesh-Like Carbon Fibers Bonding Short Carbon Fibersbeing Two-Dimensional or Three-Dimensional

The determination of the mesh-like carbon fibers bonding the shortcarbon fibers being two-dimensional or three-dimensional can beconducted by carrying out cross section observation of a sheet-likemeasurement target (three-dimensional structure Y-1, three-dimensionalstructure Y-2), and measuring the angle in the cross section between therespective carbon fibers constituting the mesh-like carbon fibersbonding the short carbon fibers (carbon fibers 2 illustrated in FIG. 7,carbon fibers 3 illustrated in FIG. 8), and the sheet surface. It shouldbe noted that the cross section in which cross section observation isperformed is a cross section in a vertical direction relative to thesheet surface of the sheet-like measurement target. (Hereinafter,“carbon fibers constituting the mesh-like carbon fibers bonding shortcarbon fibers” will be referred to as “mesh-like constituent carbonfibers”.)

A case in which the average of the angle of the mesh-like constituentcarbon fibers with a horizontal plane measured being at least 3° isdetermined as three-dimensional, while a case of the average of theangle of the mesh-like constituent carbon fibers with a horizontal planemeasured being less than 2° is determined as two-dimensional. Morespecifically, using an SEM image of a cross section in the verticaldirection relative to the sheet surface and drawing dotted linessimilarly to the dotted lines in FIGS. 5 and 6 on the mesh-likeconstituent carbon fibers being measured, it is sufficient to measurethe angles between these lines and the sheet surface, similar to themeasurement for the presence of a three-dimensional entangled structure.It should be noted that the number of measurement points upon decidingthe average value of the angle can be set to 50 points, for example.

Short Carbon Fiber (A)

As the short carbon fiber (A), one produced by cutting a carbon fibersuch as polyacrylonitrile-based carbon fiber (hereinafter referred to as“PAN-based carbon fiber”), pitch-based carbon fiber and rayon-basedcarbon fiber to an appropriate length can be exemplified. From theviewpoint of the mechanical strength of the porous electrode substrate,PAN carbon fiber is preferable. The average fiber length of the shortcarbon fibers (A) is preferably on the order of 2 to 12 mm in terms ofdispersivity. The average fiber diameter of the short carbon fibers (A)is preferably 3 to 9 μm in terms of the dispersivity of the short carbonfibers, and is more preferably 4 to 8 μm in terms of the smoothness ofthe porous electrode substrate.

Carbon (D)

The carbon (D) is used in order to bind between the short carbon fibers(A), and a carbide can be used as the carbon (D). As the carbide, acarbonaceous material obtained by carbonizing a highly polymerizedcompound by heating can be used. The form of the carbon (D) is notparticularly limited. Between the short carbon fibers (A) describedlater may be bound by carbon having a mesh-like form, between the shortcarbon fibers (A) may be bound by a carbonized resin, and it is alsopossible to use a combination of these. In addition, in a case of thecarbon (D) being a carbonized resin, it is possible to use a heatcarbonizable resin (f) as the source material thereof.

This heat carbonizable resin (f) can be selected as appropriate fromknown resins that can bind the between short carbon fibers (A) in thecarbonizing stage. From the viewpoint of facilitating remaining as aconductive material after carbonization, a phenolic resin, epoxy resin,furan resin, pitch or the like is preferred as the resin (f), and aphenolic resin having a high carbonization rate upon carbonizing byheating is particularly preferable. As the phenolic resin, a resol-typephenolic resin obtained by the reaction between phenols and aldehydesunder the presence of an alkali catalyst can be used. In addition,although a phenolic resin of Novolak type exhibiting solid-state heatfusability, produced by reaction between phenols and aldehydes under thepresence of an acid catalyst by a known method, can be dissolved andmixed into a liquid phenolic resin of resol type, in this case, one ofself crosslinking type containing a curing agent, e.g., hexamethylenediamine, is preferable. A phenolic resin solution produced by dissolvingin alcohol or a solution of ketones, a phenolic resin dispersionproduced by dispersing in a dispersant such as water, or the like can beused as the phenolic resin.

Three-Dimensional Mesh-Like Carbon Fiber (B)

The three-dimensional mesh-like carbon fibers (B) are fibers bonding theshort carbon fibers (A), and can form a three-dimensional mesh-likestructure by existing in a state forming a bent shape or a curved shapeat the bonding part.

Two-Dimensional Mesh-Like Carbon Fiber (C)

The two-dimensional mesh-like carbon fibers (C) are fibers bonding theshort carbon fibers (A), exist in a state forming a bent shape or curvedshape at the bonding part, and can form a mesh-like structure formedwithin a two-dimensional plane.

Process of Producing Porous Electrode Substrate

The porous electrode substrate of the present invention can be producedby the following such processes, for example.

The first production process is a method of sequentially performing:

a step (1) of producing a precursor sheet X-1 not having athree-dimensional entangled structure, in which the short carbon fibers(A1) are dispersed;a step (2) of obtaining a precursor sheet X-2 having a three-dimensionalentangled structure by entanglement treating the precursor sheet X-1;a step (3) of obtaining a porous electrode substrate precursor sheet X-4by layer stacking and integrating, on the precursor sheet X-2, aprecursor sheet X-3 not having a three-dimensional entangled structurein which the short carbon fibers (A-2) are dispersed; anda step (4) of carbonization treating the porous electrode substrateprecursor sheet X-4 at a temperature of at least 1000° C.

According to this production process, it is possible to obtain a porouselectrode substrate in which the three-dimensional structure Y-1 havinga three-dimensional entangled structure in which the short carbon fibers(A1) are bonded by the carbon (D), and the three-dimensional structureY-2 not having a three-dimensional entangled structure in which theshort carbon fibers (A2) are bonded by the carbon (D), are layer stackedand integrated.

In this production process, it is preferable to disperse the shortcarbon fiber precursors (b1) and/or fibrillar carbon fiber precursors(b1′) together with the short carbon fibers (A1) in step (1), and todisperse short carbon fiber precursors (b2) and/or fibrillar carbonfiber precursors (b2′) together with the short carbon fibers (A2) instep (3). It should be noted that, instead of the above, it may beimpregnated with the heat carbonizable resin (f) prior to step (4), orin addition to the above, it may be impregnated with the heatcarbonizable resin (f) prior to step (4). These fibers (b1), (b1′), (b2)and (b2′) as well as the resin (f) can function as the carbon (D) in theporous electrode substrate, after passing through carbonizationtreatment.

A second production process is a method of further performing a step (5)of hot press molding the porous electrode substrate precursor sheet X-4at a temperature less than 200° C. after step (3), but before step (4)in the above first production process.

A third production process is a method of further performing a step (6)of oxidization treating the hot press molded porous electrode substrateprecursor sheet X-4 at a temperature of at least 200° C. and less than300° C. after step (5) but before step (4) in the above secondproduction process.

It should be noted that, in the above-mentioned production processes, itis preferable to disperse the short carbon fiber precursors (b1) and/orfibrillar carbon fiber precursors (b1′) together with the short carbonfibers (A1) in step (1), and to disperse the short carbon fiberprecursors (b2) and/or fibrillar carbon fiber precursors (b2′) togetherwith the short carbon fibers (A2) in step (3). In this way, it ispossible to obtain a precursor sheet X-2′ having a three-dimensionalentangled structure in which the short carbon fibers (A1) as well as theshort carbon fiber precursors (b1) and/or fibrillar carbon fiberprecursors (b1′) are dispersed in step (1), and it is possible to obtaina precursor sheet X-3′ not having a three-dimensional entangledstructure in which the short carbon fibers (A2) as well as the shortcarbon fiber precursors (b2) and/or fibrillar carbon fiber precursors(b2′) are dispersed in step (3). It should be noted that the precursorsheet X-2′ has a three-dimensional entangled structure due to beingentanglement treated in step (2).

It should be noted that the short carbon fiber precursors (p′) and shortcarbon fiber precursors (b2) (hereinafter collectively called “shortcarbon fiber precursors (b)”) may be the same or may be different. Inaddition, the fibrillar carbon fiber precursors (b1′) and the fibrillarcarbon fiber precursors (b2′) (hereinafter collectively called“fibrillar carbon fiber precursors (b′)”) may be the same or may bedifferent.

Resin Impregnation

In a case of the carbon (D) being a carbonized resin, it is possible toestablish the porous electrode substrate by impregnating the heatcarbonizable resin (f) into the porous electrode substrate precursorsheet X-4, subsequently curing by heating and pressurizing, and thencarbonizing.

As the method of impregnating the carbonizable resin (f) into theprecursor sheet, a method using a throttling device or a methodoverlapping a resin film on the precursor sheet is preferable. Themethod using a throttling device is a method configured so as toimpregnate the precursor sheet with a resin solution, and then have thebeam limiting device uniformly coat the entire carbon sheet with theuptake liquid, and adjusting liquid amount by changing the roll gap ofthe throttling device. In a case of the relative viscosity being low, itis possible to use a spray method or the like.

The method using a resin film first temporarily coats the carbonizableresin (f) onto mold release paper to make a film of the carbonizableresin (f). It is a method that subsequently performs a hot pressingprocess to laminate the film onto the precursor sheet and transcribesthe carbonizable resin (f).

Short Carbon Fiber Precursors (b)

The short carbon fiber precursors (b) can be obtained by cutting longfibers of the carbon fiber precursor to an appropriate length. The fiberlength of the short carbon fiber precursors (b) is preferably on theorder of 2 to 20 mm in terms of dispersibility. Although thecross-sectional shape of the short carbon fiber precursors (b) is notparticularly limited, a shape having high circularity is preferable interms of the mechanical strength after carbonizing, and the productioncost. In addition, the diameter of the short carbon fiber precursors (b)is preferably no more than 5 μm in order to suppress fracture fromshrinking during carbonization.

A polymer can be used as the material of the short carbon fiberprecursors (b), and it is preferable to use a polymer having a residualmass of at least 20% by mass in the step of carbonization treatment.Acrylic polymers, cellulose-based polymers, and phenolic polymers can beexemplified as such a polymer. When taking account of the aspects ofspinnability, the ability to bond short carbon fibers (A) from lowtemperature to high temperature, and the residual mass duringcarbonization being high, and further, the fiber elasticity and fiberstrength upon performing entanglement treatment described later, it ispreferable to use an acrylic polymer containing at least 50% by massacrylonitrile units.

The short carbon fiber precursors (b) may use one type independently, ormay jointly use a plurality of types having different fiber diametersand polymers. Depending on the types of these short carbon fiberprecursors (b) and fibrillar carbon fiber precursors (b′), the mixingratio with the short carbon fibers (A) and the presence of oxidationtreatment (step (6)) under at least 200° C. and no higher than 300° C.,the proportion remaining as the three-dimensional mesh-like carbonfibers (B) or two-dimensional mesh-like carbon fibers (C) in the porouselectrode substrate finally obtained will differ.

Fibrillar Carbon Fiber Precursors (b′)

As the fibrillar carbon fiber precursors (b′), for example, fibersproduced by beating treating a carbon precursor fiber (b′-1) having astructure in which a plurality of fibrils having a diameter of tens ofmicrometers (e.g., 0.1 to 3 μm) branches from a fibrous stem with adiameter on the order of 0.1 to 10 μm (hereinafter may be simplyreferred to as “fibers (b′-1)”), and short carbon fiber precursors(b′-2) made into fibrils by beating (hereinafter may be simply referredto as “fibers (b′-2)”) can be exemplified.

By using this fibrillar carbon fiber precursor (b′), the short carbonfibers (A) and fibrillar carbon fiber precursors (b′) are wellintertwined inside the precursor sheet, whereby obtaining a precursorsheet excelling in handling property and mechanical strength isfacilitated. Although the freeness of the fibrillar carbon fiberprecursors (b′) is not particularly limited, generally, the mechanicalstrength of the precursor sheet improves when using fibrillar fibershaving low freeness; however, there is a tendency for the gaspermeability of the porous electrode substrate to decline.

As the fibrillar carbon fiber precursors (b′), one type of fiber (b′-1)or one type produced by beating treatment of the fiber (b′-2) may beused, and a plurality of types of these fibers having differentfreeness, fiber diameter, polymer type, etc. may be jointly used. Inother words, two or more types of fibers (b′-1) can be jointly used, twoor more types of fibers produced by beating treating fibers (b′-2) canbe jointly used, or alternatively, it is possible to jointly use atleast one type of fiber (b′-1) and at least one type of fiber producedby beating treating the fiber (b′-2).

The polymer used in the fiber (b′-1) preferably has a residual mass inthe carbonization treatment step of at least 20% by mass. Acrylicpolymers, cellulose polymers and phenolic polymers can be exemplified assuch a polymer. When taking account of the aspects of spinnability, theability to bond short carbon fibers (A) from low temperature to hightemperature, and the residual mass during carbonization being high, andfurther, the entangling with short carbon fibers (A) and sheet strength,it is preferable to use an acrylic polymer containing at least 50% bymass acrylonitrile units. Although the method of producing the fiber(b′-1) is not particularly limited, it is preferable to produce using aspray coagulation method for which the control of freeness is easy.

It is preferable for the average fiber length of the fibers (b′-1) to be1 to 20 mm.

Fibers produced by cutting an easy-to-split sea-island composite fiberin long fiber form to an appropriate length can be used as the fibers(b′-2). Such fibers can be beat with a refiner, pulper or the like tomake into fibrils. The fiber (b′-2) can be produced using at least twodifferent kinds of polymers that are immiscible dissolved in a commonsolvent, and in this case, at least one type of polymer preferably has aresidual mass in the carbonization treatment step of at least 20% bymass. As one having a residual mass of at least 20% by mass in thecarbonization treatment step among the polymers used in theeasy-to-split sea-island composite fibers, acrylic polymers,cellulose-based polymers and phenolic polymers can be exemplified. Fromthe viewpoint of spinnability and residual mass in the carbonizationtreatment step, thereamong, it is preferable to use an acrylic polymercontaining at least 50% by mass acrylonitrile units.

As an acrylic polymer that can be used in the fiber (b) and fiber (b′),it may be a homopolymer of acrylonitrile or a copolymer of acrylonitrileand another monomer. As the monomer copolymerized with acrylonitrile, itis not particularly limited so long as being an unsaturated monomerconstituting a general acrylic fiber; however, for example, acrylatestypified by methyl acrylate, ethyl acrylate, isopropyl acrylate, n-butylacrylate, 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate,hydroxypropylacrylate, and the like; methacrylates typified by methylmethacrylate, ethyl methacrylate, isopropyl methacrylate, n-butylmethacrylate, isobutyl methacrylate, t-butyl methacrylate, n-hexylmethacrylate, cyclohexyl methacrylate, lauryl methacrylate,2-hydroxyethyl methacrylate, hydroxypropyl methacrylate,diethylaminoethyl methacrylate, and the like; acrylic acid, methacrylicacid, maleic acid, itaconic acid, acrylamide, N-methylolacrylamide,diacetoneacrylamide, styrene, vinyltoluene, vinyl acetate, vinylchloride, vinylidene chloride, vinylidene bromide, vinyl fluoride,vinylidene fluoride, and the like can be exemplified.

The weight-average molecular weight of the acrylic polymer is notparticularly limited, and is preferably 50,000 to 1,000,000. There is atendency for the yarn quality of the fiber to be good simultaneouslywith the spinnability improving, by the weight-average molecular weightof the acrylic polymer being at least 50,000. There is a tendency forthe polymer concentration attributing to the optimum viscosity of thespinning dope to rise and the productivity to improve, by theweight-average molecular weight of the acrylic polymer being no morethan 1,000,000.

Among polymers used in the easy-to-split sea-island composite fiber thatis the fiber (b′-2), in the case of using the aforementioned acrylicpolymer as the polymer having a residual mass in the carbonizationtreatment step of at least 20% by mass, it is desirable to dissolveanother polymer in a common solvent with this acrylonitrile-basedpolymer, and the spinning dope in which both polymers are dissolved toexist stably. In other words, in a case of the extent of immiscibilityof the two polymers in the spinning dope being large, the fibers may beof uneven quality, and fiber formation may not be possible due to threadbreakage during spinning. Therefore, in the case of dissolving the otherpolymer in a common solvent with the acrylonitrile-based polymer, apolymer is desirable that has enough compatibility to be able to form asea-island structure upon spinning, but is immiscible in theacrylonitrile-based polymer. In addition, in a case of wet spinning, ifthe other polymer dissolves in water in a solidification tank or in awashing tank, loss occurs leading to disadvantages in production;therefore, it is preferable for the other polymer to be insoluble inwater.

As the other polymer satisfying these requirements, for example,polyvinyl chloride, polyvinylidene chloride, polyvinylidene fluoride,polyvinylpyrrolidone, cellulose acetate, acrylic resins, methacrylicresins, phenolic resins, etc. can be exemplified; however, celluloseacetate, acrylic resin and methacrylic resins are preferable in theaspect of the balance of the aforementioned requirements. The otherpolymer may be one type or may be two or more types.

The easy-to-split sea-island composite fiber used as the fiber (b′-2)can be produced by a common wet spinning method. First, the spinningdope is prepared by dissolving the acrylonitrile-based polymer and theother polymer in a solvent. Alternatively, the spinning dope may be madeby mixing, in a static mixer or the like, the spinning dope obtained bydissolving the acrylonitrile-based polymer in solvent and the spinningdope obtained by dissolving the other polymer in solvent. As thesolvent, dimethylamide, dimethylformamide, dimethylsulfoxide or the likecan be used. The easy-to-split sea-island composite fiber can beobtained by supplying these spinning dopes to a spinning machine andspinning from nozzles, then conducting wet hot drawing, washing, dryingand dry hot drawing.

The cross-sectional shape of the fiber (b′-2) is not particularlylimited. From the viewpoint of dispersibility, the fineness of the fiber(b′-2) is preferably 1 to 10 dtex, in order to suppress breakage due toshrinking during carbonization. The average fiber length of the fiber(b′-2) is preferably 1 to 20 mm from the viewpoint of dispersibilityafter beating.

The fiber (b′-2) is beaten by peeling off a phase-separated interfacewith a mechanical external force, and at least a portion thereof splitsto form fibrils. The beating method is not particularly limited;however, it is possible to form fibrils by a refiner or pulper, abeater, or spraying of a pressurized water stream (water-jet punching).Upon beating the fibers (b′-2) by peeling off of the phase-separatedinterface with a mechanical external force, the state of fibrillationchanges depending on the beating method and beating duration. As amethod of evaluating the extent of fibrillation, the freeness evaluation(ISO-5267-2 (Canadian Standard Freeness Method)) can be used. Thefreeness of the fibers (b′-2) is not particularly limited.

Precursor Sheet without Three-Dimensional Entangled Structure

The precursor sheet X-1 can be obtained by dispersing the short carbonfibers (A1) and the short carbon fiber precursors (b1) and/or fibrillarcarbon fiber precursors (b1′), and does not have a three-dimensionalentangled structure. The precursor sheet X-3 can be obtained bydispersing the short carbon fibers (A2) and the short carbon fiberprecursors (b2) and/or fibrillar carbon fiber precursors (b2′), and doesnot have a three-dimensional entangled structure. The short carbonfibers (A) are dispersed within a two-dimensional plane. In other words,the short carbon fibers (A1) in the precursor sheet X-1 are dispersedwithin a two-dimensional plane, and the short carbon fibers (A2) in theprecursor sheet X-3 are dispersed within a two-dimensional plane.

The mass ratio of the short carbon fibers (A1) to the short carbon fiberprecursors (b1) and fibrillar carbon fiber precursors (b1′) in theprecursor sheet X-1 is preferably 20:80 to 80:20 from the viewpoint ofensuring porous electrode substrate handling property after thecarbonization treatment. The mass ratio of the short carbon fibers (A2)to the short carbon fiber precursors (b2) and fibrillar carbon fiberprecursors (b2′) in the precursor sheet X-3 is preferably 20:80 to 80:20from the viewpoint of ensuring porous electrode substrate handlingproperties after the carbonization treatment.

The precursor sheets X-1 and X-3 can be produced by a wet method or drymethod. The wet method is a method of sheet forming a precursor sheet bydispersing the short carbon fibers (A) with the short carbon fiberprecursors (b) and/or fibrillar carbon fiber precursors (b′) in a liquidmedium. The dry method is a method of obtaining a precursor sheet bydispersing the short carbon fibers (A) with the short carbon fiberprecursors (b) and/or fibrillar carbon fiber precursors (b′) in air andallowing to fall. In order to assist in the short carbon fibers (A)opening into monofilaments, and prevent the opened monofilament frombundling again, and further, in order for the short carbon fibers (A)and short carbon fiber precursors (b) to intertwine to improve the sheetstrength and make substantially binder free, it is preferable to producethe precursor sheet by the wet method using the fibrillar carbon fiberprecursors (b′).

As the medium dispersing the short carbon fibers (A) with the shortcarbon fiber precursors (b) and/or fibrillar carbon fiber precursors(b′), a medium in which the fiber short carbon fiber precursors (b)and/or fibrillar carbon fiber precursors (b′) do not dissolve such aswater and alcohol can be exemplified, for example; however, water ispreferable from the viewpoint of productivity.

The precursor sheets X-1 and X-3 can be produced by either a continuousmethod or batch method; however, it is preferable to produce by acontinuous method from the viewpoint of the productivity and mechanicalstrength of the precursor sheets X-1 and X-3.

Entanglement Treatment

The precursor sheet X-2 is obtained by subjecting the precursor sheetX-1 to entanglement treatment. The entanglement treatment toentanglement the short carbon fibers (A) with the short carbon fiberprecursors (b) and/or fibrillar carbon fiber precursors (b′) in theprecursor sheet X-1 can be implemented by a known method so long asbeing a method in which a three-dimensional entangled structure isformed. For example, a mechanical entangling method such as a needlepunching method, a high-pressure liquid spray method such as a water-jetpunching method, a high-pressure gas spraying method such as steam-jetpunching, or a method by a combination of these can be used. Ahigh-pressure liquid jet processing method is preferable from theaspects of being able to suppress breakage of the short carbon fibers(A) in the entangling step, and sufficient entangling being obtained.

High-Pressure Liquid Jet Processing Method

The high-pressure liquid jet processing method is a processing method inwhich a precursor sheet is placed on a support member with asubstantially smooth surface, and entangling the short carbon fibers (A)with the short carbon fiber precursors (b) and/or fibrillar carbon fiberprecursors (b′) in the precursor sheet by causing a columnar liquid jet,fan-shaped liquid jet, slit liquid jet or the like sprayed at a pressureof 1 MPa to act thereon, for example. Herein, for the support memberwith a substantially smooth surface, any support member can be used solong as the design of the support member is not formed in the obtainedstructure having a three-dimensional entangled structure, and thesprayed liquid is quickly removed therefrom. As a specific examplethereof, a 30 to 200 mesh wire net or plastic net, a roll, or the likecan be exemplified.

In the present invention, from the viewpoint of productivity, it ispreferable to continuously produce the precursor sheet X-2 having athree-dimensional entangled structure by high-pressure liquid jetprocessing, after having continuously produced the precursor sheet X-1on the support member with a substantially smooth surface.

As the liquid used in the high-pressure liquid jet processing, althoughany solvent that does not dissolve the fibers constituting the precursorsheet X-1 is acceptable, it is usually preferable to use water. Thereare no particular restrictions to the temperature of the water sprayed,and room temperature water may be used. The orifice size of therespective spray nozzles in the high-pressure liquid jet nozzle ispreferably in the range of 0.06 to 1.0 mm, and more preferably in therange of 0.1 to 0.3 mm in the case of being a columnar stream. Thedistance between the nozzle jet orifice and the precursor sheet X-1 ispreferable in the range of about 0.5 to 5 cm. The pressure of the liquidis preferably at least 1 MPa and no more than 7 MPa, and more preferablyat least 1 MPa and no more than 5 MPa. The entanglement treatment may beperformed in one line, or may be performed in a plurality of lines. Inthe case of performing in a plurality of lines, it is effective toincrease the pressure of the high-pressure liquid jet processing of thesecond and later lines over the first line.

The entanglement treatment of the precursor sheet X-1 by a high-pressureliquid jet may be repeated a plurality of times. In other words, afterhaving performed the high-pressure liquid jet processing of theprecursor sheet X-1, another precursor sheet X-1 on the precursor sheetX-1 subjected to high-pressure spray processing is layer stacked, andthen the high-pressure liquid jet processing may be performed. Theprecursor sheet X-1 may be turned over while the three-dimensionalentangled structure is being formed by the high-pressure liquid jet, andthe high-pressure liquid jet processing may be further performed fromthe opposite side. In addition, these operations may be repeated.

In a case of continuously producing the precursor sheet X-2, theformation of striped track patterns resulting from the high-pressureliquid jet processing of the sheet in the sheet-forming direction can besuppressed by causing the high-pressure liquid jet nozzles provided withone line or a plurality of lines of nozzle orifices to oscillate in thewidth direction of the sheet. By suppressing the striped track patternsin the sheet-forming direction, it is possible to realize mechanicalstrength in the sheet width direction. In addition, in a case of using aplurality of the high-pressure liquid jet nozzles provided with one lineor a plurality of lines of nozzle orifices, it is possible to suppressperiodic design appearing on the three-dimensional entangled structureprecursor sheet by controlling the frequency at which the high-pressureliquid jet nozzles oscillate in the width direction of the sheet, aswell as the phase difference thereof.

Lamination and Integration

The porous electrode substrate precursor sheet X-4 is produced by layerstacking and integrating the precursor sheet X-3 not having athree-dimensional entangled structure on the precursor sheet X-2 havinga three-dimensional entangled structure. As the method of layer stackingand integrating, a method of separately producing the precursor sheetX-2 and the precursor sheet X-3, respectively, and then overlapping, amethod of directly producing the precursor sheet X-3 on the precursorsheet X-2, and the like can be exemplified. Due to the bonding betweenthe precursor sheet X-2 and the precursor sheet X-3 being easy, andfurther, the bonding force between sheets being strong, the method ofdirectly producing the precursor sheet X-3 on the precursor sheet X-2 ispreferable. For example, in the case of sheet forming by the wet method,the porous electrode substrate precursor sheet X-4 in which theprecursor sheet X-2 having a three-dimensional entangled structure andthe precursor sheet X-3 not having a three-dimensional entangledstructure are layer stacked and integrated can be obtained by directlyfeeding, onto the precursor sheet X-2 produced in advance, a slurry inwhich the short carbon fibers (A2) as well as the short carbon fiberprecursors (b2) and/or fibrillar carbon fiber precursors (b2′) aredispersed in a liquid medium to form a sheet. The porous electrodesubstrate precursor sheet X-4 can also be obtained by layer stacking aplurality of the precursor sheets X-3 on the precursor sheet X-2.

The basis weight of the porous electrode substrate precursor sheet X-4is preferably on the order of 10 to 200 g/m², and the thickness ispreferably on the order of 20 to 400 μm. It should be noted that thebasis weight of the precursor sheet X-3 not having a three-dimensionalentangled structure is preferably no more than 70% that of the porouselectrode substrate precursor sheet X-4 in the aspect of raising thehandling property of the porous electrode substrate precursor sheet X-4and the porous electrode substrate, and is preferably at least 20% thatof the porous electrode substrate precursor sheet X-4 in the aspect ofreducing the damage to the polyelectrolyte membrane upon incorporatinginto the fuel cell of the porous electrode substrate. In other words,the basis weight of the precursor sheet X-2 having a three-dimensionalentangled structure is preferably 30 to 80% that of the porous electrodesubstrate precursor sheet X-4.

Carbonization Treatment

The porous membrane base material precursor sheet X-4 can becarbonization treated as is, can be carbonization treated after hotpress molding, and can be carbonization treated after oxidationtreatment following the hot press molding. The production cost can becurbed by carbonization treating as is. The mechanical strength andconductivity of the obtained porous electrode substrate can be raised bycausing the short carbon fibers (A) to fuse by the short carbon fiberprecursors (b) and/or fibrillar carbon fiber precursors (b′1), andestablishing a three-dimensional mesh-like carbon fibers (B) ortwo-dimensional mesh-like carbon fibers (C) by carbonizing the shortcarbon fiber precursors (b) and/or fibrillar carbon fiber precursors(b′).

The carbonization treatment is preferably performed in inert gas inorder to raise the conductivity of the porous electrode substrate. Thecarbonization treatment is usually performed at a temperature of atleast 1000° C. The temperature range of the carbonization treatment ispreferably 1000 to 3000° C., and more preferably 1000 to 2200° C. Thetime of the carbonization treatment is on the order of 10 minutes to 1hour, for example. In addition, a pretreatment can be performed prior tothe carbonization treatment by calcining in an inert atmosphere on theorder of 300 to 800° C.

In a case of carbonization treating the porous electrode substrateprecursor sheet X-4 produced continuously, it is preferable to performcarbonization treatment continuously over the entire length of theporous electrode substrate precursor sheet X-4, from the viewpoint of aproduction cost reduction. If the porous electrode substrate is long,since the handling property will improve, the productivity of the porouselectrode substrate will rise, and the subsequent production of amembrane electrode assembly (MEA) can also be performed continuously, itis possible to reduce the production cost of the fuel cell. From theviewpoints of the productivity of the porous electrode substrate andfuel cell and a production cost reduction, it is preferable tocontinuously roll up the produced porous electrode substrate.

Hot Press Molding

From the viewpoint of fusing the short carbon fibers (A) in the porouselectrode substrate precursor sheet X-4 by the short carbon fiberprecursors (b) and/or fibrillar carbon fiber precursors (b′), reducingthe thickness irregularity of the porous electrode substrate, andfurther, suppressing fluffing of each fiber in the vicinity of a sheetedge surface due to the entanglement treatment, it is preferable to hotpress mold the porous electrode substrate precursor sheet X-4 at atemperature less than 200° C. prior to the carbonization treatment. Thehot press molding can adopt any technique so long as being a techniquethat can uniformly hot press mold the porous electrode substrateprecursor sheet X-4. For example, a method of hot pressing by placingflat rigid plates against both sides of the porous electrode substrateprecursor sheet X-4, and a method using a continuous roll press machineor a continuous belt press machine can be exemplified.

In the case of hot press molding the porous electrode substrateprecursor sheet X-4 continuously produced, a method using a continuousroll press machine or a continuous belt press machine is preferable.Continuously performing carbonization treatment is thereby facilitated.As the pressing method of the continuous belt press machine, a methodapplying pressure by with linear pressure to the belt by way of a rollpress, a method of pressing with specific pressure by way of a hydraulichead press, and the like can be exemplified. The latter is preferable inthe aspect of a smoother porous electrode substrate being obtained.

In order to effectively smooth the surface of the porous electrodesubstrate precursor sheet X-4, the temperature during hot press moldingis preferably less than 200° C., and more preferably 120 to 190° C.

Although the pressure during the hot press molding is not particularlylimited, it is preferably on the order of 20 kPa to 10 MPa from theviewpoint of short carbon fiber (A) breakage prevention during hot pressmolding, and the viewpoint of porous electrode substrate densificationprevention. In a case of the content ratio of the short carbon fiberprecursors (b) and/or fibrillar carbon fiber precursors (b′) in theporous electrode substrate precursor sheet X-4 being large, the surfaceof the precursor sheet Y can be easily smoothened even if the moldingpressure is low.

The duration of the hot press molding can be set to 30 seconds to 10minutes, for example. When hot press molding the porous electrodesubstrate precursor sheet X-4 by sandwiching between two rigid plates orwith a continuous roll press machine or continuous belt press machine,it is preferable to spread mold release agent thereon beforehand, or tointerpose mold release paper between the precursor sheet and the rigidplates, roll or belt, so that the short carbon fiber precursors (b)and/or fibrillar carbon fiber precursors (b′), etc. do not adhere to therigid plates, roll or belt.

Oxidation Treatment

From the viewpoint of the fusion of the short carbon fibers (A) by theshort carbon fiber precursors (b) and/or fibrillar carbon fiberprecursors (b′) being satisfactorily carried out, and improving thecarbonization rate of the short carbon fiber precursors (b) and/orfibrillar carbon fiber precursors (b′), it is preferable to oxidationtreat the porous electrode substrate precursor sheet X-4 obtained by hotpress molding, at a temperature of at least 200° C. and less than 300°C. in the atmosphere. The oxidation treatment is more preferablyperformed at 240 to 270° C.

Continuous oxidation treatment by way of a pressurized direct heatingusing a heated porous plate, or continuous oxidation treatment by way ofintermittent pressurized direct heating using a heated roller or thelike is preferable in the aspect of being low cost and being able tofuse the short carbon fibers (A) with the short carbon fiber precursors(b) and/or fibrillar carbon fiber precursors (b′).

The duration of the oxidation treatment can be set to 1 minute to 2hours, for example. In the case of oxidation treating the porouselectrode substrate precursor sheet X-4 produced continuously, it ispreferable to perform oxidation treatment continuously over the entirelength of the porous electrode substrate precursor sheet X-4.Continuously performing carbonization treatment is thereby facilitated,and it is possible to improve the productivity of the porous electrodesubstrate, membrane electrode assembly and fuel cell, and reduceproduction cost.

Membrane Electrode Assembly (MEA)

The porous electrode substrate of the present invention can be suitablyused in a membrane electrode assembly. Membrane electrode assemblies arecomposed of a polymer electrolyte membrane, catalyst layer and porouscarbon electrode base material, providing a cathode-side catalyst layercomposed of an oxidizing gas catalyst on one side of the polymerelectrolyte membrane having proton conductivity, and providing ananode-side catalyst layer composed of a fuel gas catalyst on the otherside, and in which a cathode-side porous electrode substrate andanode-side porous electrode substrate are provided on the outer sides ofthe respective catalyst layers. In order to reduce the damage from theporous electrode substrate on the polymer electrolyte membrane, it ispreferable to arrange a three-dimensional structure Y-2 side of theporous electrode substrate not having a three-dimensional entangledstructure at a surface contacting the polymer electrolyte membrane.

Polymer Electrolyte Fuel Cell

The membrane-electrode assembly of the present invention can be suitablyused in a polymer electrolyte fuel cell. The polymer electrolyte fuelcell includes a cathode-side separator on which cathode-side gaschannels are formed, and an anode-side separator on which anode-side gaschannels are formed, so as to sandwich the membrane electrode assembly.In addition, an oxidizing gas inlet and oxidizing gas outlet, and fuelgas inlet and fuel gas outlet are provided to the respective separators.

According to the present invention, it is possible to obtain a porouselectrode substrate excelling in handling property, having improvedsheet undulation, as well as retaining sufficient gas permeability andelectrical conductivity, and further, does not inflict damage on thepolymer electrolyte membrane upon incorporating in the fuel cell. Inaddition, according to the method of producing a porous electrodesubstrate of the present invention, it is possible to produce the porouselectrode substrate at low cost.

EXAMPLES

Hereinafter, the present invention will be explained more specificallyby way of the Examples. Each property value, etc. in the Examples wasmeasured by the following methods.

“Parts” indicates “parts by mass”.

(1) Gas Permeability

Based on ISO-5636-5, the time required for 200 mL of air to permeate wasmeasured using a Gurley Densometer, and the gas permeability(ml/hr/cm²/Pa) of the porous electrode substrate was calculated.

(2) Thickness

The thickness of the porous electrode substrate was measured using adial thickness gauge (trade name: 7321, manufacturing by MitutoyoCorp.). The size of the gauge head was 10 mm in diameter, and themeasurement pressure was set to 1.5 kPa.

(3) Through-Plane Resistance

The resistance value was measured when sandwiching the porous electrodesubstrate between gold plated copper plates, pressurizing from above andbelow the copper plates at 1 MPa, and flowing current at an currentdensity of 10 mA/cm², and the electrical resistance (through-planeresistance) in the thickness direction of the porous electrode substratewas obtained from the following equation. Through-plane resistance(mΩ·cm²)=measured resistance value (mΩ)×sample surface area (cm²)

(4) Total Content of Three-Dimensional Mesh-Like Carbon Fiber (B) andTwo-Dimensional Mesh-Like Carbon Fiber (C)

The total content of the three-dimensional mesh-like carbon fiber (B)and two-dimensional mesh-like carbon fiber (C) was calculated accordingto the following formula from the basis weight of the obtained porouselectrode substrate and the basis weight of the short carbon fibers (A)used.

Total content (mass %) of three-dimensional mesh-like carbon fiber (B)and two-dimensional mesh-like carbon fiber (C)=[porous electrodesubstrate basis weight (g/m²)−short carbon fiber (A) basis weight(g/m²)]/porous electrode substrate basis weight (g/m²)×100

(5) Undulation of Porous electrode substrate

The undulation of the porous electrode substrate was calculated from thedifference between the maximum value and minimum value of the heightwhen laying a porous electrode substrate with a height of 250 mm andwidth of 250 mm on a flat plate.

(6) Damage to Polymer Electrolyte Membrane when Incorporating to FuelCell

A perfluorosulfonic acid-based polymer electrolyte membrane (membranethickness: 30 μm) on which catalyst layers (catalyst layer surface area:25 cm², Pt deposit per unit area: 0.3 mg/cm²) composed of catalystloaded carbon (catalyst: Pt, catalyst loading: 50% by mass) were formedon both sides was sandwiched between two porous electrode substrates sothat the three-dimensional structure side not having a three-dimensionalentangled structure contacted with the polymer electrolyte membrane, andthese were bonded to obtain an MEA. This MEA was interposed by the twocarbon separators having bellows-like gas channels to prepare a polymerelectrolyte fuel cell (unit cell). Then, by measuring the open circuitvoltage (OCV) when supplying hydrogen gas and air through bubblers at80° C. to the unit cell with the temperature set to 80° C., the damageto the polymer electrolyte membrane upon incorporating in the fuel cellwas confirmed.

Example 1

PAN-based carbon fiber having an average fiber diameter of 7 μm andaverage fiber length of 3 mm was prepared as the short carbon fiber (A).In addition, as the short carbon fiber precursor (b), acrylic shortfiber having an average fiber diameter of 4 μm and average fiber lengthof 3 mm was prepared (trade name: D122, manufactured by Mitsubishi RayonCo., Ltd.), and as the fibrillar carbon fiber precursor (b′),easy-to-split acrylic sea-island composite fiber (b′-2) composed ofdiacetate (cellulose acetate) and acrylic polymer to be fibrillated bybeating was prepared (trade name: VONNEL M.V.P-C651, average fiberlength: 3 mm, manufactured by Mitsubishi Rayon Co., Ltd.)

By the following operations, the precursor sheet X-1, precursor sheetX-2 having a three-dimensional entangled structure, and the porouselectrode substrate precursor sheet X-4 produced by layer stacking andintegrating the precursor sheet X-3 not having a three-dimensionalentangled structure onto the precursor sheet X-2 were continuouslyproduced to obtain a carbon electrode base material.

(Defibration of Short Carbon Fibers (A))

The short carbon fibers (A) were dispersed in water so that the fiberconcentration was 1% by mass (10 g/L), and were defibration treatedthrough a disk refiner (manufactured by Kumagai Riki Kogyo Co., Ltd.) toprovide defibrated slurry fibers (SA).

(Defibration of Short Carbon Fiber Precursors (b))

The short carbon fiber precursors (b) were dispersed in water so thatthe fiber concentration was 1% by mass (10 g/L), and were defibrationtreated through a disk refiner (manufactured by Kumagai Riki Kogyo Co.,Ltd.) to provide defibrated slurry fibers (Sb).

(Defibration of Fibrillar Carbon Fiber Precursors (b′))

The easy-to-split acrylic sea-island composite short fibers weredispersed in water so that the fiber concentration was 1% by mass (10g/L), and were subjected to beating and defibration treatment through adisk refiner (manufactured by Kumagai Riki Kogyo Co., Ltd.) to providedefibrated slurry fibers (Sb').

(Preparation of Sheet-Forming Slurry)

The defibrated slurry fibers (SA), defibrated slurry fibers (Sb),defibrated slurry fibers (Sb′) and dilution water were measured so thatthe mass ratio of the short carbon fibers (A) to short carbon fiberprecursors (b) to fibrillar carbon fiber precursors (b′) was 50:30:20and the concentration of fibers in the slurry was 1.44 g/L, and werecharged into a slurry feed tank. Furthermore, polyacrylamide was addedto prepare a sheet-forming slurry with a viscosity of 22 centipoise (22mPa·s).

(Production of Precursor Sheet X-1)

Treatment equipment was used including a sheet-shaped material conveyingdevice made from a net driving unit, and a continuously rotatable net inwhich a plain-woven mesh made of a 60 cm wide by 585 cm long plastic netwas connected in a belt shape; a sheet-forming slurry feed apparatushaving a slurry feed portion width of 48 cm and a feed slurry amount of30 L/min; and a reduced-pressure dewatering apparatus arranged under thenet.

Then, the aforementioned sheet-forming slurry was fed above theplain-woven mesh by a metering pump. The sheet-forming slurry waswidened to a predetermined size through a flow box for rectifying to auniform flow, and then fed. Subsequently, it was left to stand, passedthrough a natural dewatering portion, and then dewatered by thereduced-pressure dewatering apparatus, thereby obtaining the precursorsheet X-1. It should be noted that the target basis weight of theprecursor sheet X-1 was set to 35 g/m².

(Production of Precursor Sheet X-2 Having Three-Dimensional EntangledStructure)

Downstream of the treatment equipment, the pressurized water stream jettreatment apparatus including three water jet nozzles shown in the belowTable 1 was arranged.

TABLE 1 Pitch between orifices Nozzle effective Orifice size (widthdirection) Arrangement width Nozzle1 φ 0.15 mm × 501 orifices 1 mm 1line arrangement 500 mm (1001 orifices/1 m width) Nozzle2 φ 0.15 mm ×501 orifices 1 mm 1 line arrangement 500 mm (1001 orifices/1 m width)Nozzle3 φ 0.15 mm × 1002 orifices 1.5 mm 3 line arrangement 500 mm Pitchbetween lines 5 mm

The precursor sheet X-1 was loaded onto a net of the pressurized waterstream jet treatment apparatus. Then, setting the pressurized waterstream jet pressure to 1 MPa (nozzle 1), 2 MPa (nozzle 2) and 1 MPa(nozzle 3), the precursor sheet X-1 was subjected to entanglementtreatment by being passed in the order of the nozzle 1, nozzle 2 andnozzle 3, thereby obtaining the precursor sheet X-2 having athree-dimensional entangled structure. It should be noted that thetarget basis weight of the precursor sheet X-2 having athree-dimensional entangled structure is 35 g/m², which is the same asthe target basis weight of the precursor sheet X-1.

(Production of Porous Electrode Substrate Precursor Sheet X-4 in whichPrecursor Sheet X-3 without Three-Dimensional Entangled Structure isLayer Stacked and Integrated on Precursor Sheet X-2)

Further downstream of the pressurized water stream jet treatmentapparatus, treatment equipment (treatment equipment including sheet-likematerial conveying apparatus, sheet-forming slurry feed apparatus, andreduced-pressure dewatering apparatus arranged under net) similar to thetreatment equipment used in the production of the precursor sheet X-1were arranged.

Then, the aforementioned sheet-forming slurry was fed by a metering pumpfrom above the precursor sheet X-2 having a three-dimensional entangledstructure loaded onto the plain-woven mesh. The sheet-forming slurry waswidened to a predetermined size through a flow box for rectifying to auniform flow, and then fed. Subsequently, it was left to stand, passedthrough a natural dewatering portion, and dewatered by thereduced-pressure dewatering apparatus, and the precursor sheet X-3 nothaving a three-dimensional entangled structure was layer stacked,thereby obtaining the porous electrode substrate precursor sheet X-4 inwhich the precursor sheet X-2 having a three-dimensional entangledstructure and the precursor sheet X-3 not having a three-dimensionalentangled structure are layer stacked and integrated. It should be notedthat, since the target basis weight of the precursor sheet X-3 was setto 35 g/m², the target basis weight of the porous electrode substrateprecursor sheet X-4 is 70 g/m².

(Dry Treatment)

The porous electrode substrate precursor sheet X-4 was dried for 3minutes at 150° C. by a pin tenter tester (trade name: PT-2A-400,manufactured by Tsuji Dyeing Machine Manufacturing Co., Ltd.). The basisweight of the porous electrode substrate precursor sheet X-4 was 70.2g/m². The dispersed state of the short carbon fibers (A), short carbonfiber precursors (b) and fibrillar carbon fiber precursors (b′) in thisporous electrode substrate precursor sheet X-4 was favorable, andfurther, e of fibers in the precursor sheet X-2 was favorable, and thehandling property was also favorable.

(Hot Press Molding)

Next, both surfaces of the porous electrode substrate precursor sheetX-4 were interposed between paper coated with a silicone-based moldrelease agent, and then, was hot press molded for 3 minutes underconditions of 180° C. and 3 MPa in a batch press machine.

(Carbonization Treatment)

Subsequently, the porous electrode substrate precursor sheet X-4 wascarbonization treated in a batch carbonization furnace in a nitrogen gasatmosphere under the condition of 2000° C. to obtain the porouselectrode substrate.

(Evaluation Results)

The obtained porous electrode substrate had almost no in-plane shrinkingduring the carbonization treatment, the undulation was small at no morethan 2 mm and thus the surface smoothness was favorable, and the gaspermeability, thickness and through-plane electric resistance were eachfavorable. The total content of the three-dimensional mesh-like carbonfibers (B) and two-dimensional mesh-like carbon fibers (C) was 24% bymass. In addition, scanning electron micrographs of cross sections ofthe obtained porous electrode substrate are shown in FIG. 1 (crosssection of three-dimensional structure having three-dimensionalentangled structure) and FIG. 2 (cross section of three-dimensionalstructure not having three-dimensional entangled structure). Inaddition, scanning electron micrographs of front and back surfaces ofthe obtained porous electrode substrate are shown in FIGS. 3 and 4. InFIG. 1, it could be confirmed that the short carbon fibers (A) arebonded by the three-dimensional mesh-like carbon fibers (B), and in FIG.2, it could be confirmed that the short carbon fibers (A) are bonded bythe two-dimensional mesh-like carbon fibers (C). Since the A surfacedoes not have an entangled structure in FIG. 3, it could be confirmedthat fluffing of the short carbon fibers (A), carbonized acrylic fibers,is suppressed. On the other hand, since the B surface has an entangledstructure, fibers projecting from the surface could be observed in FIG.4 (locations with round border). The OCV of a unit cell using thisporous electrode substrate was high at 0.902 V, and the damage to thepolymer electrolyte membrane upon incorporating in the fuel cell waslow. The above evaluation results are shown in Table 2 along with thebasis weight of the porous electrode substrate.

Examples 2 and 3

Porous electrode substrates were obtained similarly to Example 1, exceptfor setting the target basis weights of the precursor sheet X-2 having athree-dimensional entangled structure and the precursor sheet X-3 nothaving a three-dimensional entangled structure to 25 g/m² and 45 g/m²(Example 2), or 55 g/m² and 15 g/m² (Example 3), respectively. Theobtained porous electrode substrate had almost no in-plane shrinkingduring the carbonization treatment, the undulation of the sheet wassmall at no more than 2 mm and thus the surface smoothness wasfavorable, and the gas permeability, thickness and through-planeelectric resistance were each favorable. In addition, the porouselectrode substrate had a structure integrating and layer stacking athree-dimensional structure in which the short carbon fibers (A) arebonded by the three-dimensional mesh-like carbon fibers (B), and athree-dimensional structure in which the short carbon fibers (A) arebonded by the two-dimensional mesh-like carbon fibers (C). In addition,the damage to the polymer electrolyte membrane upon incorporating in thefuel cell was also small. The above evaluation results are shown inTable 2.

Examples 4 and 5

Porous electrode substrates were obtained similarly to Example 1, exceptfor setting the target basis weights of the precursor sheet X-2 having athree-dimensional entangled structure, the precursor sheet X-3 nothaving a three-dimensional entangled structure, and the porous electrodesubstrate precursor sheet X-4 layer stacking and integrating these to 30g/m², 30 g/m² and 60 g/m² (Example 4), or 20 g/m², 20 g/m² and 40 g/m²(Example 5), respectively. The obtained porous electrode substrate hadalmost no in-plane shrinking during the carbonization treatment, theundulation of the sheet was small at no more than 2 mm and thus thesurface smoothness was favorable, and the gas permeability, thicknessand through-plane electric resistance were each favorable. In addition,the porous electrode substrate had a structure integrating and layerstacking a three-dimensional structure in which the short carbon fibers(A) are bonded by the three-dimensional mesh-like carbon fibers (B), anda three-dimensional structure in which the short carbon fibers (A) arebonded by the two-dimensional mesh-like carbon fibers (C). In addition,the damage to the polymer electrolyte membrane upon incorporating in thefuel cell was also small. The above evaluation results are shown inTable 2.

Example 6

A porous electrode substrate was obtained similarly to Example 1, exceptfor setting the mass ratio of the short carbon fibers (A) to shortcarbon fiber precursors (b) to fibrillar carbon fiber precursors (b′) inthe sheet-forming slurry to 50:40:10. The obtained porous electrodesubstrate had almost no in-plane shrinking during the carbonizationtreatment, the undulation of the sheet was small at no more than 2 mmand thus the surface smoothness was favorable, and the gas permeability,thickness and through-plane electric resistance were each favorable. Inaddition, the porous electrode substrate had a structure integrating andlayer stacking a three-dimensional structure in which the short carbonfibers (A) are bonded by the three-dimensional mesh-like carbon fibers(B), and a three-dimensional structure in which the short carbon fibers(A) are bonded by the two-dimensional mesh-like carbon fibers (C). Inaddition, the damage to the polymer electrolyte membrane uponincorporating in the fuel cell was also small. The above evaluationresults are shown in Table 2.

Example 7

A porous electrode substrate was obtained similarly to Example 1, exceptfor setting the mass ratio of the short carbon fibers (A) to shortcarbon fiber precursors (b) to fibrillar carbon fiber precursors (b′) inthe sheet-forming slurry to 40:40:20, and setting the target basisweights of the precursor sheet X-2 having a three-dimensional entangledstructure, the precursor sheet X-3 not having a three-dimensionalentangled structure, and the porous electrode substrate precursor sheetX-4 layer stacking and integrating these to 40 g/m², 40 g/m² and 80g/m², respectively. The obtained porous electrode substrate had almostno in-plane shrinking during the carbonization treatment, the undulationof the sheet was small at no more than 2 mm and thus the surfacesmoothness was favorable, and the gas permeability, thickness andthrough-plane electric resistance were each favorable. In addition, theporous electrode substrate had a structure integrating and layerstacking a three-dimensional structure in which the short carbon fibers(A) are bonded by the three-dimensional mesh-like carbon fibers (B), anda three-dimensional structure in which the short carbon fibers (A) arebonded by the two-dimensional mesh-like carbon fibers (C). In addition,the damage to the polymer electrolyte membrane upon incorporating in thefuel cell was also small. The above evaluation results are shown inTable 2.

Example 8

A porous electrode substrate was obtained similarly to Example 1, exceptfor setting the mass ratio of the short carbon fibers (A) to shortcarbon fiber precursors (b) to fibrillar carbon fiber precursors (b′) inthe sheet-forming slurry to 30:50:20, and setting the target basisweights of the precursor sheet X-2 having a three-dimensional entangledstructure, the precursor sheet X-3 not having a three-dimensionalentangled structure, and the porous electrode substrate precursor sheetX-4 layer stacking and integrating these to 45 g/m², 45 g/m² and 90g/m², respectively. The obtained porous electrode substrate had almostno in-plane shrinking during the carbonization treatment, the undulationof the sheet was small at no more than 2 mm and thus the surfacesmoothness was favorable, and the gas permeability, thickness andthrough-plane electric resistance were each favorable. In addition, theporous electrode substrate had a structure integrating and layerstacking a three-dimensional structure in which the short carbon fibers(A) are bonded by the three-dimensional mesh-like carbon fibers (B), anda three-dimensional structure in which the short carbon fibers (A) arebonded by the two-dimensional mesh-like carbon fibers (C). In addition,the damage to the polymer electrolyte membrane upon incorporating in thefuel cell was also small. The above evaluation results are shown inTable 2.

Example 9

A porous electrode substrate was obtained similarly to Example 1, exceptfor setting the mass ratio of the short carbon fibers (A) to shortcarbon fiber precursors (b) to fibrillar carbon fiber precursors (b′) inthe sheet-forming slurry to 70:10:20, and setting the target basisweights of the precursor sheet X-2 having a three-dimensional entangledstructure, the precursor sheet X-3 not having a three-dimensionalentangled structure, and the porous electrode substrate precursor sheetX-4 layer stacking and integrating these to 30 g/m², 30 g/m² and 60g/m², respectively.

The obtained porous electrode substrate had almost no in-plane shrinkingduring the carbonization treatment, the undulation of the sheet wassmall at no more than 2 mm and thus the surface smoothness wasfavorable, and the gas permeability, thickness and through-planeelectric resistance were each favorable. In addition, the porouselectrode substrate had a structure integrating and layer stacking athree-dimensional structure in which the short carbon fibers (A) arebonded by the three-dimensional mesh-like carbon fibers (B), and athree-dimensional structure in which the short carbon fibers (A) arebonded by the two-dimensional mesh-like carbon fibers (C). In addition,the damage to the polymer electrolyte membrane upon incorporating in thefuel cell was also small. The above evaluation results are shown inTable 2.

Example 10

A porous electrode substrate was obtained similarly to Example 1, exceptfor setting the pressurized water stream jet pressure to 2 MPa (nozzle1), 3 MPa (nozzle 2) and 2 MPa (nozzle 3). The obtained porous electrodesubstrate had almost no in-plane shrinking during the carbonizationtreatment, the undulation of the sheet was small at no more than 2 mmand thus the surface smoothness was favorable, and the gas permeability,thickness and through-plane electric resistance were each favorable. Inaddition, the porous electrode substrate had a structure integrating andlayer stacking a three-dimensional structure in which the short carbonfibers (A) are bonded by the three-dimensional mesh-like carbon fibers(B), and a three-dimensional structure in which the short carbon fibers(A) are bonded by the two-dimensional mesh-like carbon fibers (C). Inaddition, the damage to the polymer electrolyte membrane uponincorporating in the fuel cell was also small. The above evaluationresults are shown in Table 2.

Example 11

A porous electrode substrate was obtained similarly to Example 1, exceptfor setting the pressurized water stream jet pressure to 3.5 MPa (nozzle1), 4.5 MPa (nozzle 2) and 3.5 MPa (nozzle 3). The obtained porouselectrode substrate had almost no in-plane shrinking during thecarbonization treatment, the undulation of the sheet was small at nomore than 2 mm and thus the surface smoothness was favorable, and thegas permeability, thickness and through-plane electric resistance wereeach favorable. In addition, the porous electrode substrate had astructure integrating and layer stacking a three-dimensional structurein which the short carbon fibers (A) are bonded by the three-dimensionalmesh-like carbon fibers (B), and a three-dimensional structure in whichthe short carbon fibers (A) are bonded by the two-dimensional mesh-likecarbon fibers (C). In addition, the damage to the polymer electrolytemembrane upon incorporating in the fuel cell was also small. The aboveevaluation results are shown in Table 2.

Example 12

A porous electrode substrate was obtained similarly to Example 1, exceptfor using a polyacrylonitrile-based pulp (b′-1) in which several fibrilshaving a diameter of no more than 3 μm branch from a fibrous stem as thefibrillar carbon fiber precursor (b′). It should be noted that thepolyacrylonitrile-based pulp (b′-1) was produced by jet solidification.The obtained porous electrode substrate had almost no in-plane shrinkingduring the carbonization treatment, the undulation of the sheet wassmall at no more than 2 mm and thus the surface smoothness wasfavorable, and the gas permeability, thickness and through-planeelectric resistance were each favorable. In addition, the porouselectrode substrate had a structure integrating and layer stacking athree-dimensional structure in which the short carbon fibers (A) arebonded by the three-dimensional mesh-like carbon fibers (B), and athree-dimensional structure in which the short carbon fibers (A) arebonded by the two-dimensional mesh-like carbon fibers (C). In addition,the damage to the polymer electrolyte membrane upon incorporating in thefuel cell was also small. The above evaluation results are shown inTable 2.

Example 13

A porous electrode substrate was obtained similarly to Example 12,except for repeatedly conducting the three-dimensional entanglementtreatment by way of a pressurized water stream jet twice from the samesurface. The obtained porous electrode substrate had almost no in-planeshrinking during the carbonization treatment, the undulation of thesheet was small at no more than 2 mm and thus the surface smoothness wasfavorable, and the gas permeability, thickness and through-planeelectric resistance were each favorable. In addition, the porouselectrode substrate had a structure integrating and layer stacking athree-dimensional structure in which the short carbon fibers (A) arebonded by the three-dimensional mesh-like carbon fibers (B), and athree-dimensional structure in which the short carbon fibers (A) arebonded by the two-dimensional mesh-like carbon fibers (C). In addition,the damage to the polymer electrolyte membrane upon incorporating in thefuel cell was also small. The above evaluation results are shown inTable 2.

Example 14

A porous electrode substrate was obtained similarly to Example 12,except for implementing the pressurized water stream jet again from theback surface after conducting the three-dimensional entanglementtreatment by way of the pressurized water stream jet from the topsurface. The obtained porous electrode substrate had almost no in-planeshrinking during the carbonization treatment, the undulation of thesheet was small at no more than 2 mm and thus the surface smoothness wasfavorable, and the gas permeability, thickness and through-planeelectric resistance were each favorable. In addition, the porouselectrode substrate had a structure integrating and layer stacking athree-dimensional structure in which the short carbon fibers (A) arebonded by the three-dimensional mesh-like carbon fibers (B), and athree-dimensional structure in which the short carbon fibers (A) arebonded by the two-dimensional mesh-like carbon fibers (C). In addition,the damage to the polymer electrolyte membrane upon incorporating in thefuel cell was also small. The above evaluation results are shown inTable 2.

Example 15

A porous electrode substrate was obtained similarly to Example 1, exceptfor not using the fibrillar carbon fiber precursors (b′), and settingthe mass ratio of the short carbon fibers (A) to short carbon fiberprecursors (b) in the sheet-forming slurry to 50:50. The obtained porouselectrode substrate had almost no in-plane shrinking during thecarbonization treatment, the undulation of the sheet was small at nomore than 2 mm and thus the surface smoothness was favorable, and thegas permeability, thickness and through-plane electric resistance wereeach favorable. In addition, the porous electrode substrate had astructure integrating and layer stacking a three-dimensional structurein which the short carbon fibers (A) are bonded by the three-dimensionalmesh-like carbon fibers (B), and a three-dimensional structure in whichthe short carbon fibers (A) are bonded by the two-dimensional mesh-likecarbon fibers (C). In addition, the damage to the polymer electrolytemembrane upon incorporating in the fuel cell was also small. The aboveevaluation results are shown in Table 2.

Example 16

A porous electrode substrate was obtained similarly to Example 1, exceptfor not using the short carbon fiber precursors (b), and setting themass ratio of the short carbon fibers (A) to fibrillar carbon fiberprecursors (b′) in the sheet-forming slurry to 50:50. The obtainedporous electrode substrate had almost no in-plane shrinking during thecarbonization treatment, the undulation of the sheet was small at nomore than 2 mm and thus the surface smoothness was favorable, and thegas permeability, thickness and through-plane electric resistance wereeach favorable. In addition, the porous electrode substrate had astructure integrating and layer stacking a three-dimensional structurein which the short carbon fibers (A) are bonded by the three-dimensionalmesh-like carbon fibers (B), and a three-dimensional structure in whichthe short carbon fibers (A) are bonded by the two-dimensional mesh-likecarbon fibers (C). In addition, the damage to the polymer electrolytemembrane upon incorporating in the fuel cell was also small. The aboveevaluation results are shown in Table 2.

Example 17

A porous electrode substrate was obtained similarly to Example 16,except for using a polyacrylonitrile-based pulp (b′-1) in which severalfibrils having a diameter of no more than 3 μm branch from a fibrousstem as the fibrillar carbon fiber precursor (b′). It should be notedthat the polyacrylonitrile-based pulp (b′-1) was produced by jetsolidification. The obtained porous electrode substrate had almost noin-plane shrinking during the carbonization treatment, the undulation ofthe sheet was small at no more than 2 mm and thus the surface smoothnesswas favorable, and the gas permeability, thickness and through-planeelectric resistance were each favorable. In addition, the porouselectrode substrate had a structure integrating and layer stacking athree-dimensional structure in which the short carbon fibers (A) arebonded by the three-dimensional mesh-like carbon fibers (B), and athree-dimensional structure in which the short carbon fibers (A) arebonded by the two-dimensional mesh-like carbon fibers (C). In addition,the damage to the polymer electrolyte membrane upon incorporating in thefuel cell was also small. The above evaluation results are shown inTable 2.

Example 18

A porous electrode substrate was obtained similarly to Example 1, exceptfor interposing both surfaces of a hot press molded porous electrodesubstrate precursor sheet X-4 with stainless-steel punching platescoated with a silicone-based mold release agent, and then oxidationtreating in a batch press machine in the atmosphere under conditions of280° C. and 0.5 MPa, prior to the carbonization treatment. The obtainedporous electrode substrate had almost no in-plane shrinking during thecarbonization treatment, the undulation of the sheet was small at nomore than 2 mm and thus the surface smoothness was favorable, and thegas permeability, thickness and through-plane electric resistance wereeach favorable. In addition, the porous electrode substrate had astructure integrating and layer stacking a three-dimensional structurein which the short carbon fibers (A) are bonded by the three-dimensionalmesh-like carbon fibers (B), and a three-dimensional structure in whichthe short carbon fibers (A) are bonded by the two-dimensional mesh-likecarbon fibers (C). In addition, the damage to the polymer electrolytemembrane upon incorporating in the fuel cell was also small. The aboveevaluation results are shown in Table 2.

Example 19

A porous electrode substrate was obtained similarly to Example 1, exceptfor not having carried out hot press molding. The obtained porouselectrode substrate had almost no in-plane shrinking during thecarbonization treatment, the undulation of the sheet was small at nomore than 2 mm and thus the surface smoothness was favorable, and thegas permeability, thickness and through-plane electric resistance wereeach favorable. In addition, the porous electrode substrate had astructure integrating and layer stacking a three-dimensional structurein which the short carbon fibers (A) are bonded by the three-dimensionalmesh-like carbon fibers (B), and a three-dimensional structure in whichthe short carbon fibers (A) are bonded by the two-dimensional mesh-likecarbon fibers (C). In addition, the damage to the polymer electrolytemembrane upon incorporating in the fuel cell was also small. The aboveevaluation results are shown in Table 2.

Example 20

A porous electrode substrate was obtained similarly to Example 1, exceptfor setting the mass ratio of the short carbon fibers (A) to shortcarbon fiber precursors (b) to fibrillar carbon fiber precursors (b′) inthe sheet-forming slurry to 20:30:50, and setting the target basisweights of the precursor sheet X-2 having a three-dimensional entangledstructure, the precursor sheet X-3 not having a three-dimensionalentangled structure, and the porous electrode substrate precursor sheetX-4 layer stacking and integrating these to 45 g/m², 45 g/m² and 90g/m², respectively. The obtained porous electrode substrate had anexternal appearance in which wrinkles were formed due to in-planeshrinking during the carbonization treatment; however, the undulationwas small at 3 mm, the surface smoothness was also favorable, and thegas permeability, thickness and through-plane electric resistance wereeach favorable. In addition, the porous electrode substrate had astructure integrating and layer stacking a three-dimensional structurein which the short carbon fibers (A) are bonded by the three-dimensionalmesh-like carbon fibers (B), and a three-dimensional structure in whichthe short carbon fibers (A) are bonded by the two-dimensional mesh-likecarbon fibers (C). In addition, the damage to the polymer electrolytemembrane upon incorporating in the fuel cell was also small. The aboveevaluation results are shown in Table 2.

Example 21

A porous electrode substrate was obtained similarly to Example 1, exceptfor setting the mass ratio of the short carbon fibers (A) to shortcarbon fiber precursors (b) to fibrillar carbon fiber precursors (b′) inthe sheet-forming slurry to 80:10:10, and setting the target basisweights of the precursor sheet X-2 having a three-dimensional entangledstructure, the precursor sheet X-3 not having a three-dimensionalentangled structure, and the porous electrode substrate precursor sheetX-4 layer stacking and integrating these to 30 g/m², 30 g/m² and 60g/m², respectively.

The obtained porous electrode substrate had an external appearance inwhich wrinkles were formed due to in-plane shrinking during thecarbonization treatment; however, the undulation was small at 3 mm, thesurface smoothness was also favorable, and the gas permeability,thickness and through-plane electric resistance were each favorable. Inaddition, the porous electrode substrate had a structure integrating andlayer stacking a three-dimensional structure in which the short carbonfibers (A) are bonded by the three-dimensional mesh-like carbon fibers(B), and a three-dimensional structure in which the short carbon fibers(A) are bonded by the two-dimensional mesh-like carbon fibers (C). Inaddition, the damage to the polymer electrolyte membrane uponincorporating in the fuel cell was also small. The above evaluationresults are shown in Table 2.

Example 22

A porous electrode substrate was obtained similarly to Example 1, exceptfor producing the layer stacked and integrated precursor sheet byseparately producing the precursor sheet X-2 having a three-dimensionalentangled structure and the precursor sheet X-3 not having athree-dimensional entangled structure, allowing to dry, and thenoverlapping the two and hot press molding in a batch press machine for 3minutes under conditions of 180° C. and 3 MPa. It should be noted thatthe precursor sheet X-3 was produced similarly to the production methodof X-1 in Example 1. The obtained porous electrode substrate had anexternal appearance in which wrinkles were formed due to in-planeshrinking during the carbonization treatment, the undulation was smallat no more than 2 mm, the surface smoothness was also favorable, and thegas permeability, thickness and through-plane electric resistance wereeach favorable. In addition, the porous electrode substrate had astructure integrating and layer stacking a three-dimensional structurein which the short carbon fibers (A) are bonded by the three-dimensionalmesh-like carbon fibers (B), and a three-dimensional structure in whichthe short carbon fibers (A) are bonded by the two-dimensional mesh-likecarbon fibers (C). In addition, the damage to the polymer electrolytemembrane upon incorporating in the fuel cell was also small. The aboveevaluation results are shown in Table 2.

Example 23

In place of the defibrated slurries Sb and Sb′ of the short carbon fiberprecursors (b) and fibrillar carbon fiber precursors (b′) a defibratedslurry was used that had been prepared similarly to the defibratedslurry Sb using polyvinyl alcohol (PVA) short fibers having an averagefiber length of 3 mm (trade name: VBP105-1, manufactured by Kuraray Co.,Ltd.). Then, the mass ratio of the short carbon fibers (A) to thepolyvinyl alcohol (PVA) short fibers in the sheet-forming slurry was setto 80:20. Otherwise, the porous electrode substrate precursor sheet X-4was obtained similarly to Example 5.

Subsequently, the porous electrode substrate precursor sheet X-4impregnated by phenolic resin was obtained by impregnating the porouselectrode substrate precursor sheet X-4 with a methanol solution of thephenolic resin (trade name: Phenolite J-325, manufactured by DainipponInk and Chemicals, Inc.), and allowing the methanol to sufficiently dryat room temperature, so that the mass ratio of the porous electrodesubstrate precursor sheet X-4 to nonvolatile components of the phenolicresin was 50:50. Subsequently, press-heat molding and carbonizationtreatment were performed at the same conditions of Example 1 to obtain aporous electrode substrate. The obtained porous electrode substrate hadalmost no in-plane shrinking during the carbonization treatment, theundulation of the sheet was small at no more than 2 mm and thus thesurface smoothness was favorable, and the gas permeability, thicknessand through-plane electric resistance were each favorable. In addition,the porous electrode substrate had a structure integrating and layerstacking a three-dimensional structure having an entangled structure inwhich the short carbon fibers (A) are bonded by the carbon (D), and athree-dimensional structure not having an entangled structure in whichthe short carbon fibers (A) are bonded by the carbon (D). In addition,the damage to the polymer electrolyte membrane upon incorporating in thefuel cell was also small. The above evaluation results are shown inTable 2.

Example 24

(Production of Membrane-Electrode Assembly)

An MEA was obtained similarly to the technique described in theaforementioned OCV measurement method (evaluation method for damage tothe polymer electrolyte membrane upon incorporating in the fuel cell),using two of the porous electrode substrates obtained in Example 1 asthe porous carbon electrode base materials for the cathode and anode.

(Evaluation of Fuel Cell Characteristics of MEA)

The obtained MEA was interposed by two carbon separators havingbellows-like gas channels to form a polymer electrolyte fuel cell (unitcell).

Evaluation of the fuel cell characteristics was performed by measuringthe current density-voltage characteristics of this unit cell. Hydrogengas was used as the fuel gas and air was used as the oxidizing gas. Thetemperature of the unit cell was set to 80° C., the fuel gas utilizationrate was set to 60%, and the oxidizing gas utilization rate was set to40%. In addition, humidification of the fuel gas and oxidizing gas wasperformed by passing the fuel gas and the oxidizing gas through bubblersat 80° C., respectively. As a result, the cell voltage of the fuel cellwhen the current density was 0.8 A/cm² was 0.644 V, and the internalresistance of the cell was 2.9 mΩ, which indicated favorablecharacteristics.

Comparative Example 1

A porous electrode substrate was obtained similarly to Example 1, exceptfor not having conducted the three-dimensional entanglement treatment byway of a pressurized water stream jet. In other words, although theprecursor sheet X-3 was formed on the precursor sheet X-1, since neitherhas a three-dimensional entangled structure, the target basis weights ofboth were totaled and indicated in the field of “basis weight of X-3” inTable 2.

The obtained porous electrode substrate had almost no in-plane shrinkingduring the carbonization treatment, the undulation of the sheet wassmall at no more than 2 mm and thus the surface smoothness wasfavorable, and the gas permeability, thickness and through-planeelectric resistance were each favorable; however, the handling propertyof the porous electrode substrate precursor sheet X-4 greatly declined.In addition, three-dimensional mesh-like carbon fibers (B) were notobserved in the porous electrode substrate, which had a structure inwhich the short carbon fibers (A) were bonded by the two-dimensionalmesh-like carbon fibers (C). In addition, the damage to the polymerelectrolyte membrane upon inserting in the fuel cell was small. Theabove evaluation results are shown in Table 2.

Comparative Example 2

A porous electrode substrate was obtained similarly to Example 1, exceptfor obtaining the porous electrode substrate precursor sheet X-4 bylayer stacking and integrating the precursor sheet X-2 having athree-dimensional entangled structure and the precursor sheet X-3 nothaving a three-dimensional entangled structure, and then furtherconducting entanglement treatment by way of a pressurized water streamjet similarly to Example 1. In other words, since there was the resultof the precursor sheet X-3 also having a two-dimensional entangledstructure in addition to the precursor sheet X-2, the target basisweight of both were totaled and indicated in the field of “basis weightof X-2” in Table 2.

The obtained porous electrode substrate had almost no in-plane shrinkingduring the carbonization treatment, the undulation of the sheet wassmall at no more than 2 mm and thus the surface smoothness wasfavorable, and the gas permeability, thickness and through-planeelectric resistance were each favorable. In addition, two-dimensionalmesh-like carbon fibers (C) were not observed in the porous electrodesubstrate, which had a structure in which the short carbon fibers (A)were bonded by the three-dimensional mesh-like carbon fibers (B).However, the OCV of the unit cell using this porous electrode substratewas low at 0.883 V, and thus the damage to the polymer electrolytemembrane upon incorporating in the fuel cell was large. The aboveevaluation results are shown in Table 2.

Comparative Example 3

A porous electrode substrate was obtained similarly to Example 1, exceptfor not using the short carbon fibers (A), and setting the mass ratio ofthe short carbon fiber precursors (b) to the fibrillar carbon fiberprecursors (b′) in the sheet-forming slurry to a mass ratio of 60:40.The obtained porous electrode substrate had a large amount of in-planeshrinkage during the carbonization treatment, and the sheet form couldnot be retained.

Comparative Example 4

In place of the defibrated slurries Sb and Sb′ of the short carbon fiberprecursors (b) and fibrillar carbon fiber precursors (b′), a defibratedslurry was used that had been prepared similarly to the defibratedslurry Sb using polyvinyl alcohol (PVA) short fibers having an averagefiber length of 3 mm (trade name: VBP105-1, manufactured by Kuraray Co.,Ltd.). Then, the mass ratio of the short carbon fibers (A) to thepolyvinyl alcohol (PVA) short fibers in the sheet-forming slurry was setto 80:20. Otherwise, the porous electrode substrate was obtainedsimilarly to Example 1. The three-dimensional mesh-like carbon fibers(B) and two-dimensional mesh-like carbon fibers (C) were not formed inthe obtained porous electrode substrate, and the sheet form could not beretained.

TABLE 2 Precursor sheet Fibrillar carbon fiber precursors (b′) Carbonfiber Short precursors carbon having fiber Short large precursors Shortcarbon number of to be carbon fiber branched fibrillated Short BasisBasis Basis fibers precursors fibrils by beating PVA weight of weight ofweight of (A) (b) (b′-1) (b′-2) fibers X-4 X-2 X-3 Parts Parts PartsParts Parts g/m² g/m² g/m² Example 1 50 30 0 20 0 70 35 35 Example 2 5030 0 20 0 70 25 45 Example 3 50 30 0 20 0 70 55 15 Example 4 50 30 0 200 60 30 30 Example 5 50 30 0 20 0 40 20 20 Example 6 50 40 0 10 0 70 3535 Example 7 40 40 0 20 0 80 40 40 Example 8 30 50 0 20 0 90 45 45Example 9 70 10 0 20 0 60 30 30 Example 10 50 30 0 20 0 70 35 35 Example11 50 30 0 20 0 70 35 35 Example 12 50 30 20 0 0 70 35 35 Example 13 5030 20 0 0 70 35 35 Example 14 50 30 20 0 0 70 35 35 Example 15 50 50 0 00 70 35 35 Example 16 50 0 0 50 0 70 35 35 Example 17 50 0 50 0 0 70 3535 Example 18 50 30 0 20 0 70 35 35 Example 19 50 30 0 20 0 70 35 35Example 20 20 30 0 50 0 90 45 45 Example 21 80 10 0 10 0 60 30 30Example 22 50 30 0 20 0 70 35 35 Example 23 80 0 0 0 20 40 20 20Comparative Example 1 50 30 0 20 0 70 0 70 Comparative Example 2 50 30 020 0 70 70 0 Comparative Example 3 0 60 0 40 0 70 35 35 ComparativeExample 4 80 0 0 0 20 70 35 35 Porous electrode substrate Total contentof three-dimensional mesh-like carbon Open circuit fibers (B) andvoltage two-dimensional (damage to Basis mesh-like carbon GasThrough-plane Electrode weight Thickness fibers (C) permeabilityresistance membrane) g/m² μm % ml/hr/cm2/Pa mΩ · cm2 V Example 1 46 20724 1100 12 0.902 Example 2 46 205 24 1100 12 0.904 Example 3 46 208 241100 11 0.900 Example 4 39 177 24 1300 10 0.903 Example 5 26 111 23 22006 0.900 Example 6 45 199 22 1000 11 0.904 Example 7 46 180 30 800 120.908 Example 8 46 166 41 700 12 0.910 Example 9 47 161 11 900 11 0.905Example 10 46 210 24 1200 11 0.903 Example 11 46 211 24 1300 11 0.902Example 12 45 201 22 900 13 0.905 Example 13 45 203 22 1000 13 0.906Example 14 45 200 22 1000 13 0.905 Example 15 45 198 23 1100 12 0.902Example 16 46 200 23 1000 12 0.904 Example 17 46 197 23 900 14 0.901Example 18 48 206 27 1100 11 0.904 Example 19 46 215 24 1500 13 0.900Example 20 41 155 56 500 12 0.911 Example 21 42 145 5 900 11 0.906Example 22 46 203 24 1000 12 0.903 Example 23 48 195 0 800 6 0.904Comparative Example 1 46 205 24 1100 12 0.903 Comparative Example 2 46209 24 1200 12 0.883 Comparative Example 3 — — — — — — ComparativeExample 4 — — — — — —

1. A porous electrode substrate comprising, layer stacked and integratedtherein: a three-dimensional structure Y-1 produced by bonding shortcarbon fibers A1 by carbon D; and a three-dimensional structure Y-2produced by bonding short carbon fibers A2 by carbon D, wherein theshort carbon fibers A1 form a three-dimensional entangled structure inthe three-dimensional structure Y-1, and the short carbon fibers A2 donot form a three-dimensional entangled structure in thethree-dimensional structure Y-2.
 2. The porous electrode substrateaccording to claim 1, wherein the three-dimensional structure Y-1 is athree-dimensional structure in which the short carbon fibers A1 arebonded by three-dimensional mesh-like carbon fibers B, and thethree-dimensional structure Y-2 is a three-dimensional structure inwhich the short carbon fibers A2 are bonded by two-dimensional mesh-likecarbon fibers C.
 3. A porous electrode substrate precursor sheetcomprising, layer stacked and integrated therein: a precursor sheet X-2′comprising dispersed therein short carbon fibers A1, and at least one ofshort carbon fiber precursors b1 and fibrillar carbon fiber precursorsb1′; and a precursor sheet X-3′ comprising dispersed therein shortcarbon fibers A2, and at least one of short carbon fiber precursors b2and fibrillar carbon fiber precursors b2′, wherein the short carbonfibers A1 form a three-dimensional entangled structure in the precursorsheet X-2′, and the short carbon fibers A2 do not form athree-dimensional entangled structure in the precursor sheet X-3′. 4.The porous electrode substrate precursor sheet according to claim 3,wherein a mass ratio of a mass of the short carbon fibers A1 to a totalmass of the short carbon fiber precursors b1 and the fibrillar carbonfiber precursors b1′ is from 20:80 to 80:20, and a mass ratio of a massof the short carbon fibers A2 to a total mass of the short carbon fiberprecursors b2 and the fibrillar carbon fiber precursors b2′ is from20:80 to 80:20.
 5. A process of producing a porous electrode substrate,the process comprising: (a) dispersing short carbon fibers A1, andproducing a precursor sheet X-1 not having a three-dimensional entangledstructure of the short carbon fibers A1; (b) obtaining a precursor sheetX-2 having a three-dimensional entangled structure of the short carbonfibers A1, by entanglement treating the precursor sheet X-1; (c)obtaining a porous electrode substrate precursor sheet X-4 by dispersingshort carbon fibers A2, and layer stacking and integrating a precursorsheet X-3 not having a three-dimensional entangled structure of theshort carbon fibers A2, on the precursor sheet X-2; and (d)carbonization treating the porous electrode substrate precursor sheetX-4 at a temperature of at least 1000° C., thereby obtaining the porouselectrode substrate.
 6. The process according to claim 5, wherein theprecursor sheet X-2 having a three-dimensional entangled structure ofthe short carbon fibers A1 and the precursor sheet X-3 not having athree-dimensional entangled structure of the short carbon fibers A2 arelayer stacked and integrated in said obtaining (c), by feeding a slurryin which the short carbon fibers A2 are dispersed in a liquid mediumonto the precursor sheet X-2 and making into a sheet.
 7. The processaccording to claim 5, wherein the short carbon fiber precursors b1,fibrillar carbon fiber precursors b1′, or both are dispersed togetherwith the short carbon fibers A1 in said dispersing (a), and the shortcarbon fiber precursors b2, fibrillar carbon fiber precursors b2′ orboth are dispersed together with the short carbon fibers A2 in saidobtaining (c).
 8. The process according to claim 5, further comprising:(e) hot press molding the porous electrode substrate precursor sheet X-4at a temperature of less than 200° C., after said obtaining (c) butbefore said carbonization treating (d).
 9. The process according toclaim 8, further comprising: (f) oxidation treating, at a temperature ofat least 200° C. and less than 300° C., the porous electrode substrateprecursor sheet X-4, after said hot press molding (e) but before saidcarbonization treating (d).
 10. A porous electrode substrate obtained bythe process according to claim
 5. 11. A membrane-electrode assemblycomprising the porous electrode substrate according to claim
 1. 12. Apolymer electrolyte fuel cell comprising the membrane-electrode assemblyaccording to claim 11.